An American Christian imprisoned in North Korea is calling for the United States to try harder to secure his freedom.

Kenneth Bae, 44, told North Korean media he is still hopeful the U.S. government will help him gain amnesty from a sentence of 15 years hard labor.

"I ask the U.S. government for help and hope it will make more efforts for my return at an early date," Bae, who hails from Washington state, said in a video produced by a North Korean news agency.

A State Department official who saw the video said there's "grave concern" over his deteriorating health. Bae is diabetic.

"We have seen the interview footage of Bae," the official said. "It is clear his health is deteriorating, which is of grave concern to us. There is no greater priority for us than the welfare and safety of U.S. citizens abroad."

Bae had hoped to be released Thursday, not only because it's America's Independence Day, but also because it's his father's 70th birthday.

Bae was arrested while leading a tour group in North Korea in early November 2012.

In April, a North Korean court convicted him of plotting to commit "hostile acts" against the government and sentenced him to 15 years hard labor.

It's widely believed his imprisonment is a political ploy by the North Korean regime.